Business jets were involved in 18 nonfatal accidents and three fatal accidents during the first nine months of this year, compared with 16 nonfatal accidents and one fatal accident in the same period last year, according to safety analyst Robert E. Breiling Associates of Boca Raton, Fla. Although the number of fatal accidents involving business jets tripled, the number of people killed remained level at eight for both periods. Meanwhile, in the first three quarters of this year, fatal accidents involving business turboprops increased to 13 from 10 last year, but fatalities decreased to 24 this year from 32 last year.